(1) Field of the Invention
The present invention relates to a cleaning device for a wafer mount plate used for polishing semiconductor wafers, and more particularly to a cleaning device for washing the whole surface of the wafer mount plate.
(2) Background of the Invention
In the fabrication of semiconductors, it is requires to make the surface of semiconductor wafers extremely smooth. In general, wafers are pasted on a mount plate by means of an adhesive, such as wax diluted by a solvent, and the mount plate is set on a polishing machine to polish the wafers. This general method is described, for example, in U.S. Pat. No. 4,960,485 issued Oct. 2, 1990. However, if the adhesive between the wafers and the mount plate is not applied thinly and uniformly, minute unevenness is formed on the surface when polishing is conducted and it becomes difficult to obtain high precision.
Further, if fine dust or other minute particles exist, on the surface of the mount plate, the adhesive tends to form dimples at the areas where the dust or particles are located thereby preventing uniform adhesion of the wafers. To avoid this problem, after completion of the polishing operation, the wafers are peeled from the mount plate, and the mount plate is thoroughly washed so that the adhesive and the like does not remain as dust.
Since the mount plate is repeatedly used for the polishing step, the mount plate is made of a material, such as ceramics or a glass, which undergoes small deformation with respect to age deterioration and load. Various kinds of disks, each having a diameter of several tens of centimeters and a thickness of about 20 millimeters, are prepared for use as mount plates so that various kinds of wafers having different sizes may be pasted thereon. The weight on these mount plates is typically from about 20 kg to 30 kg or more.
At present, as a device for cleaning the mount plate, there is known a device in which the mount plate is washed while it is being transported and before pasting the wafers thereon. Such a device comprises upper and lower laterally elongated cylindrical brush rolls extending across a conveying path of the mount plate, so that the upper and lower surfaces of the mount plate can be washed by brushing when the mount plate passes between the brush rolls. However, since the diameter of the mount plate is relatively large as mentioned above, the brush rolls must have a length longer than the diameter of the mount plate. As a consequence, if a part of the brush roll is curved or bowed even a little, it sometimes causes a part of the surface of the mount plate to remain unwashed.
Further, since the peripheral side surface (edge surface) of the mount plate can not be washed by this prior art device, the adhesive remaining on this portion tends to peel off in powdery form during mounting of the wafers upon the mount plate. The powdery adhesive adheres on the mounting surface of the plate and causes formation of dimples in the subsequent adhesive coating step.
In addition, in order to remove the adhesive by washing, it is necessary to carry out the steps of first removing the adhesive, then brushing the surface with pure water, and finally rinsing the surface. Thus, the device tends to become large in size, which is a drawback because the device can not be manufactured economically.
Another device is known in which the mount plated is washed by immersion in organic solvent baths. This device comprises a heated organic solvent bath, a normal temperature organic solvent bath, an organic solvent vapor bath, etc., wherein the mount plate is sequentially immersed in each bath to wash it, followed by drying. However, since the surface of the mount plate is not brushed, the washing or cleaning effect is insufficient. In addition, since the mount plate is heavy and large as compared with the wafers, each immersion bath is large and requires a large area for installation.
In recent years, a cleaning method that does not use organic solvent has been sought from the standpoint of reducing air pollution and for safety and hygienic reasons. It is also necessary to improve the conventional devices from these points of view.